This invention relates generally to magnetic resonance imaging (MRI), and more particularly the invention relates to separation of signals from species (e.g. water/fat) using steady state free precession (SSFP) imaging.
Magnetic resonance imaging (MRI) provides excellent soft tissue contrast with arbitrary scan-volume orientations, thus making MRI an extremely useful medical imaging modality. However, in many applications, MRI is limited by long scan times, limited spatial resolution, and contrast between lipid-based tissue and water-based tissue. Recent advances in gradient amplifier technology have enabled the use of fully-refocused steady-state free precession (SSFP) imaging methods. SSFP imaging is a very fast method that can provide good tissue contrast and high resolution. A number of commercial implementations of SSFP are available, all of which are conceptually identical.
As illustrated in FIG. 1, a refocused SSFP sequence consists of a single RF excitation which is repeated periodically. All gradients used for slice selection or imaging are fully rewound over each repetition time, TR. In the steady-state, the magnetization at points a and d is the same. Magnetization is tipped about the x-axis through an angle α. Between excitations, the magnetization undergoes a precession by an angle θ=2πΔfTR about the z-axis, where f is the tissue off-resonance, and also experiences both T1 and T2 relaxation.
During the sequence each spin is affected by RF pulses, relaxation and free precession. The steady-state magnetization for SSFP is a function of the sequence parameters flip angle (a), repetition time (TR) and echo time (TE) as well as the tissue parameters T1, T2, and off-resonant frequency Δf.
Magnetic resonance imaging (MRI) is widely used for clinical diagnosis of neurological, cardiovascular, and musculoskeletal disorders. However, these and other applications of MRI are still limited by spatial resolution, signal-to-noise ratio (SNR) and imaging speed. Balanced steady-state free precession (SSFP) provides good tissue contrast with high signal-to-noise ratio, thus addressing the latter two limitations of MRI. Historically, there have been two major difficulties regarding clinical use of balanced SSFP. The first of these is the high sensitivity of the signal to resonance frequency variations caused by static field inhomogeneity, chemical shift, and susceptibility shifts. The second difficulty is the high signal produced by fat, which can obscure visualization of normal and abnormal tissue.
Traditionally, the best approach for reducing the sensitivity to frequency variations has been to reduce the repetition time (TR) of the sequence so that precession over one TR is small, and a wider frequency variation can be tolerated. However, this is ultimately limited by a combination of gradient amplifier speed, patient stimulation effects and RF heating of the subject. An alternative to decreasing the TR has been to perform multiple acquisitions with different center frequencies.
Numerous methods have been presented to suppress the fat signal, or to separate it from the water signal. Many of these methods require multiple acquisitions, or otherwise increased scan time. Some of the faster methods can suffer from transient artifacts.
Although it suffers from partial-volume effects, one of the fastest, and most efficient fat-water separation techniques is phase-sensitive SSFP, which uses the signal phase in a standard balanced SSFP acquisition to separate water and fat. However, like other SSFP fat/water separation methods, phase-sensitive SSFP fails in the presence of significant frequency variations. The present invention is directed to overcoming this limitation.